CHROME STEELS 183
7. Detection of Silicon.—The sample is ground with sodium carbonate'
and nitre and the mass dissolved in dilute hydrochloric acid and evaporated
to dryness. After being heated for an hour in an oven at 120-130°, the
residue is treated with hot dilute hydrochloric acid and filtered. In presence
of silicon, a more or less abundant residue of silica, attackable by hydro-
fluoric acid, remains. Ferro-silicon with a high proportion of silicon is
dissolved in 10% sodium hydroxide solution.
8. Detection of Titanium.—A little of the sample is dissolved in
nitric acid and evaporated to dryness, the residue being ignited to decom-
pose nitrates. The oxides thus obtained are fused with bisulphate, the
cooled mass being dissolved in cold water, acidified with sulphuric acid
and treated with hydrogen peroxide. In presence of titanium a distinctly
yellow coloration is exhibited.
9. Detection of Aluminium.—A little of the sample is dissolved in
hydrochloric acid diluted with an equal volume of water, a few drops of
nitric acid being added (to oxidise the iron), the liquid rendered alkaline
with sodium hydroxide, diluted, boiled for some time and filtered. When
the filtrate is heated with excess of ammonium chloride, a gelatinous pre-
cipitate of aluminium hydroxide is obtained if aluminium is present.
Chrome steels are hard and highly resistant to ordinary acids, and
are used particularly for the manufacture of tools, projectiles, cylinders,
etc. Their analysis includes, besides the ordinary determinations, that of
1. Determination of the Chromium.—The commonest methods for
estimating chromium in steels consist in transforming the chromium into
chromic acid or alkaline chromate and in determining the latter volumetri-
cally. Owing to its accuracy and rapidity, the iodometric method is to
be recommended. In presence of vanadium and molybdenum, which
often accompany chromium in steels, the method is slightly modified to
get rid of these metals, which behave towards iodine as chromium does.
Reagents : (i) N/io-potassium dichromate solution, prepared by dis-
solving 4-9033 grams of the salt—recrystallised in small crystals and dried
at 130°—in water to I litre.
(2) Approximately N/io-sodium thiosulphate, 25 grams being dissolved
to i litre.
(3) Starch paste.
Titration of the thiosulphate. To establish the titre of the thiosulphate
solution with respect to the dichromate, and hence with respect to chromium,
20 c.c. of the dichromate solution (corresponding with 0-09806 gram of
dichromate) are measured out, and 10 c.c. of 10% potassium iodide (free
from iodate) 1 solution and 5 c.c. of hydrochloric acid (D ri) added, the
liquid being then shaken and, after addition of 200—250 c.c. of water, titrated
with the thiosulphate solution ; starch paste is added towards the end of
1 An aqueous solution of the potassium iodide, acidified with hydrochloric acid
and mixed with a little starch paste, should remain colourless.tion of hydrogen sulphide, a